My invention pertains to a vehicle movement detector for use with a vehicle overspeed control system. More particularly, the invention relates to a detector which checks the integrity of the vehicle speed measuring means pickup device when the vehicle is moving and provides an output signal, to enable the continued release of the vehicle brakes, only when a valid speed signal is registered.
Speed control systems for vehicles including railroad trains require apparatus for measuring the actual vehicle or train speed so that it may be compared with the desired speed. Frequently, this apparatus is of the tachometer, axle generator type which produces an alternating current signal whose frequency varies in proportion with the train speed. Integrity of the tachometer apparatus is very important because an unsafe condition can result if failure occurs, for example, if the drive shaft to the axle generator should break as the train starts to move. Therefore, some form of movement detector is required which will register the output of the tachometer or speed generator, to assure its integrity and reliability, at those times when other conditions indicate that the train is moving. In the absence of output from such a detector, a brake application is actuated. An alternate method is to use two speed pickup devices with a comparison for agreement of the outputs. However, this redundancy is costly and results in complex circuitry. Where the speed control apparatus is part of an automatic train operation system, the use of a zero velocity detector as part of the system is required to assure that door opening occurs only when the train is fully stopped at a station platform. Such zero velocity apparatus also provides a simulated tachometer signal to enable the train to start up from the station stop. An example of vital zero motion or zero speed checking apparatus for use in a train control system is disclosed in U.S. Pat. No. 3,553,488, issued Jan. 5, 1971 to John O. G. Darrow and Raymond C. Franke for a Fail-Safe Circuit Arrangement. When such zero speed checking apparatus is used with a train control system, a simple arrangement such as a high pass filter cannot be used to check tachometer integrity during train movement since the high frequency electrical check signal produced by the zero speed detector when the train is stopped is applied through the tachometer pickup element. This will result in output from the movement detector even if there is no speed signal. Since both detector arrangements are necessary in the complete system, an arrangement which inhibits this incorrect response of the movement detector to the high frequency signal from the zero speed apparatus is necessary.
Accordingly, an object of my invention is a vital vehicle movement detector for use in a speed control system in combination with a zero speed detector arrangement.
Another object of the invention is a vehicle movement detector which checks the integrity of the vehicle speed measuring apparatus when the vehicle is in motion.
A further object of my invention is a vital vehicle movement detector in which the detection of vehicle movement to register the integrity of the speed measuring device is inhibited if the associated zero velocity detector registers a vehicle at stop.
Still another object of the invention is a vehicle movement detector apparatus which is responsive to various input signals to check integrity and operation of the vehicle speed measuring axle generator device only when the train should be moving and has exceeded a preselected minimum speed level.
Yet another object of the invention is detector apparatus for use in a complete train control system, which includes a zero speed detector arrangement, to assure the integrity of the train speed pickup device only when the train is moving to enable the speed governor to retain train brakes released to continue the train movement.
Other objects, features, and advantages of my invention will become apparent from the following specification and appended claims when taken in connection with the accompanying drawing.